The present invention relates, in general, to methods of testing integrated circuits, and more particularly, to a method of testing integrated circuits which are attached to a leadframe.
To ensure functionality, integrated circuits (ICs) must be tested several times before they are shipped to a customer. Although most processes involved in manufacturing integrated circuits are batch processes in which many circuits are processed at a single time, testing has usually called for individual handling of the circuits. Integrated circuits are usually encapsulated on leadframes which comprise a plurality of circuits, but it has been necessary to separate the circuits from the leadframe to test them. Once the circuits are removed from the leadframe, they are particularly fragile, and leads which extend from the package are easily bent or torn. Stringent requirements for lead and package quality placed on manufacturers make it important to handle the packaged integrated circuits as little as possible once they are removed from the leadframe. These requirements are not congruent with the manufacturer's need to handle and test each of the integrated circuit packages individually.
IC packages are usually attached to a leadframe by the leads themselves and by tie-bars which couple package to the leadframe. The packages are trimmed from the leadframe in a trim and leadform tool which has several stages, each stage performing only part of the trim process. After the leads are trimmed so that they are mechanically separated from the leadframe and from each other, they are usually lead formed into a predetermined shape to meet a customer's specification. After the leadform is complete, the tie-bars are cut, separating the IC package from the leadframe completely. The trim and leadform operations are performed sequentially in the same tool so that damage to the leads and package resulting from handling is minimal.
At a stage in the trim/leadform process after the leads were electrically separated from the leadframe but before the packages were mechanically separated from the leadframe the ICs were electrically separate from each other so that testing could be done, yet it was impossible to make electrical contact to the package leads for testing. A major difficulty with testing devices which are still attached to a leadframe is making contact to a large number of half-formed pins or leads in the narrow confines of the trim and leadform tool. Conventional test head geometries cannot make contact in these conditions.
Although testing is designed to improve the reliability of integrated circuits, often it compromises the quality of packaged devices. Individual loading and unloading of integrated circuits into test fixtures often damaged leads which extend outside the integrated circuit package. This damage was usually not repairable and resulted in completely functional devices being rejected for physical quality problems. In addition to lead damage, package damage also occured. Also, handling of packaged integrated circuits increased the chances of damage to the circuits themselves by electrostatic discharge. Although testing is necessary to provide the desired reliability and functionality, the above mentioned quality limits have long been a costly problem for circuit manufacturers.
Accordingly, it is an object of the present invention to provide a means and method for testing integrated circuits which are attached to a leadframe.
It is another object of the present invention to provide a means and method for testing integrated circuits which reduces cycle time.
Another object of the present invention is to provide a means and method for testing integrated circuits which reduces testing cost.
A further object of the present invention is to provide a means and method for testing integrated circuits which eliminates individual handling of the integrated circuits.
Still another object of the present invention is to provide a method of testing integrated circuits which improves package quality.